1. Field of Invention
The present invention relates to a key code security arrangement that determines whether a particular key is authorized. An interrogator unit sends a key code request to a key-based transponder unit. In response to the request, the key-based transponder unit transmits a signal including its pre-assigned key code. The interrogator unit then compares the key code transmitted by the transponder unit with key codes previously stored in the interrogator unit.
2. Description of Related Art
Because convention keys are easy to duplicate, there are various kinds of key code security arrangements used in motor vehicles. It is desirable to be able to easily determine whether a particular key inserted into an ignition switch key lock is an authorized key. Various kinds of immobilizer systems are known. Such systems prevent the motor vehicle from being operated if the key is not authorized. When an attempt is made to start an engine without using a correct key, an immobilizer system may, for example, prohibit the supply of fuel to the engine, prevent ignition or otherwise interfere with the operation of the motor vehicle.
In general, such systems rely on the use of two separate units. A vehicle-side unit is, essentially, an interrogator device that sends request signals to key-based transponder units. The vehicle-side interrogator unit includes an immobilizer coil that is wrapped around a key cylinder of the ignition system. The transponder is built into the ignition key. The interrogator unit transmits a signal constituting a request. This signal is transmitted via the immobilizer coil which functions as an antenna. The request signal is received by the key-based transponder. The request signal can be used to provide operating power to the transponder and, of course, carries the request itself. In response to the request, the transponder transmits a signal including a key code previously assigned to that key and stored in a memory of the transponder.
The interrogator unit receives the transmitted key code and compares it with a plurality of previously registered reference key codes stored in a memory associated with the interrogator unit. If there is a match indicating that the key responding is authorized, the vehicle-side device permits the vehicle to be operated. For many such systems, a plurality of keys are manufactured and authorized and it is intended that any of them can be used. The immobilizer system must, therefore, be able to cope with a plurality of key codes. According to Japanese Patent Publication Laid-Open No. HEI 9-88390, a plurality of key codes are registered in the vehicle-side device. Then, according to a predetermined priority sequence, there is an attempt to correlate a received key code with one authorized code after another. In this manner, the immobilizer system can cope with a plurality of key codes.
In some cases, a plurality of keys, such as on a key ring, may be in the vicinity of the key cylinder. This situation is depicted in FIG. 7. One of two keys on the key ring is inserted into the key cylinder. However the transponders of both keys are likely to respond to a request from the interrogator unit because both are in close proximity to the immobilizer coil. Accordingly, two transponders respond to a key code request from the vehicle-side device. These responses interfere with each other and confuse the vehicle-side interrogator device.
To prevent such interference, key-specific key code requests for each authorized key can be transmitted by the interrogator unit one by one in sequence. Each key-based transponder will only respond to its key-specific request. This avoids the interference problem due to simultaneous responses by a plurality of key-based transponders. However, if there are many authorized keys, this process of sending sequential key code requests and waiting for appropriate responses can be very time consuming. This process also consumes significant power. This situation is unacceptable. When a vehicle is stopped, the process of interrogation may be repeated many times.